Thioethers of cysteine are derived metabolically from thioethers of glutathione, the latter formed as the initial reaction in mercapturic acid synthesis. The final step in this process of detoxication was thought to be acetylation to yield a thioether of N-acetylcysteine, i.e., a mercapturic acid. Although the enzyme for this last reaction has now been solubilized and identified, a shunt pathway was found that diverts thioethers of cysteine from acetylation. The shunt is formed by cysteine conjugate Beta-lyase which catalyzes the reaction in which R-S-CH2CH(NH2)C00H is cleaved to form a thiol, pyruvate and ammonia. Detailed investigation of this enzyme, now in homogneous form, from rat liver discloses that the lyase will act on a wide range of compounds that bear a good leaving group on the beta carbon of alanine. Each of the substrates is a suicide substrate in its inactivation of the enzyme. These observations have pertinence in explaining the toxicity of the environmental carcinogen produced from 1,1,2-trichloroethane.